A fuel cell is one kind of electrical generators which take out electric energy by electrochemically oxidizing a fuel such as hydrogen or methanol, and has been paid attention as a clean energy supply source, in recent years. Inter alia, since a solid polymer fuel cell has a low standard working temperature of around 100° C., and has high energy density, wide application as an electrical generator for a dispersion-type electric power generation facility at a relatively small scale or a movable body such as an automobile or a ship is expected. The fuel cell is also paid attention as an electric supply for a small movable equipment or a portable equipment, and installation into a portable telephone, a personal computer or the like, in place of secondary cells such as nickel metal hydride and a lithium ion cell, is expected.
In the fuel cell, usually, anode and cathode electrodes in which a reaction bearing electric power generation occurs, and a polymer electrolyte membrane which is to be a proton conductor between the anode and the cathode constitute a membrane electrode assembly (hereinafter, abbreviated as MEA in some cases), and the fuel cell is constituted of, as a unit, a cell in which this MEA is sandwiched by separators. The polymer electrode membrane is mainly constituted of a polymer electrolyte material. The polymer electrolyte material is also used in a binder of an electrode catalyst layer or the like. Examples of the required property of the polymer electrolyte membrane include firstly high proton conductivity, and it is necessary that the polymer electrolyte membrane has high proton conductivity particularly even under high temperature and the low humidification condition. Since the polymer electrolyte membrane bears a function as a barrier which prevents a direct reaction between a fuel and oxygen, the membrane is required to have low permeability of the fuel. In addition, examples of the required properties include chemical stability for enduring a strong oxidative atmosphere during fuel cell operation, mechanical strength and physical durability which can endure membrane thinning and repeating of swelling and drying, and the like.
Previously, in the polymer electrolyte membrane, Nafion (registered trademark) (manufactured by Du Pont) which is a perfluorosulfonic acid polymer has been widely used. Since Nafion (registered trademark) is made via multistage synthesis, there are problems that Nafion is very expensive, and has great fuel crossover. In addition, a problem that mechanical strength and physical durability of a membrane are lost due to swelling and drying, a problem that a softening point is low, and use at a high temperature is not possible and, further, a problem of disposal after use, and a problem that recycle of the material is difficult have been pointed out.
Under such circumstances, development of a hydrocarbon electrolyte membrane as a polymer electrolyte material, which can replace Nafion (registered trademark), is inexpensive and is excellent in membrane properties, has been activated in recent years.
For example, a block copolymer having a hydrophobic segment in which a sulfonic acid group has not been substantially introduced, and a hydrophilic segment in which a sulfonic acid group has been introduced, the hydrophobic segment containing polyether sulfone (PES) or polyether ketone, and the hydrophilic segment containing sulfonated polyether sulfone or sulfonated polyether ketone, has been proposed (Patent Documents 1 and 2). In the documents, as the hydrophilic segment, a constituent unit in which a sulfonic acid group has been introduced into 50% of all phenyl groups, that is, an alternate copolymer of aromatic dihalide in which two sulfonic acid groups have been introduced into two phenyl groups, and bisphenol in which a sulfonic acid group has not been introduced into two phenyl groups is used. Usually, since these PESs and polyether ketones are synthesized using an aromatic nucleophilic substitution reaction of electron withdrawing aromatic dihalide and electron donating bisphenol, introduction of an electron withdrawing sulfonic acid group is limited to an aromatic dihalide side, and it is known that it is difficult to introduce a sulfonic acid group into more than 50% of all phenyl groups. Therefore, the present inventors have considered that, in the prior art, there is limitation in further local densification of a sulfonic acid group in a hydrophilic domain, and improvement in proton conductivity under the low humidification condition.
In Patent Document 3, there is described a trial of synthesizing a mixture comprising a disulfonated product, trisulfonated product and tetrasulfonated product of 4,4′-difluorobenzophenone at 50, 30 and 20 mol %, respectively, and copolymerizing the mixture with a fluorene bisphenol. However, since a content mole ratio of the tetrasulfonated product contained in the constituent unit containing a sulfonic acid group is 20 mol %, there is limitation in improvement in low humidification proton conductivity. In the document, as far as the tetrasulfonated product is not selectively synthesized, and usually, these disulfonated product, trisulfonated product and tetrasulfonated product have similar polarity, it is difficult to separate and purify them even using chromatography or the like and, further, it is not possible to enhance a content mole ratio of the tetrasulfonated product.
In Non-Patent Document 1, there is described regarding a block copolymer containing polyether sulfone (PES) as a hydrophobic segment, and sulfonated polyether sulfone in which a sulfonic acid group has been introduced into 100% of all phenyl groups as a hydrophilic segment. The present inventors have confirmed that, in the document, after a polymer similar to that of Patent Document 1 is obtained, a sulfonic acid group is introduced into a phenyl group having high electron density adjacent to an ether group, and thus desulfonation due to a reverse reaction easily proceeds. Further, the present inventors have considered that there is a problem that chemical stability is insufficient, moreover, a post-sulfonation reaction and re-precipitation are necessary, and the number of steps is increased, and thus the cost becomes high.
Like this, the polymer electrolyte material obtained by the prior art is insufficient as a means for improving economical property, processability, proton conductivity, mechanical strength, chemical stability and physical durability, and can not be an industrially useful polymer electrolyte material.